Polyester represented by polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN), etc. is excellent in its mechanical characteristics and chemical characteristics. In addition, highly functionalized copolymerized polyesters prepared by selection of various kinds and types of acid ingredients/alcohol ingredients upon demands have been used depending upon their characteristics in broad fields including, for example, a fiber for a clothing and industrial materials; a film and a sheet for a package, a magnetic tape and optics; a bottle which is a hollow molded product; a casing for electric and electronic parts; a binder for a paint, an adhesive and an ink; and others such as an engineering plastic molded product.
Polyester has been usually produced by preparing an oligomer mixture by means of an esterification reaction or ester transfer reaction of dicarboxylic acid and/or an ester-form derivative thereof with diol and/or an ester-form derivative thereof, and by subjecting the resulting oligomer mixture to a liquid phase polycondensation using a catalyst at a high temperature in vacuo.
With regard to a catalyst used for the polycondensation of polyester as such, an antimony compound or a germanium compound and a titanium compound has/have been widely used.
An antimony trioxide is a catalyst being less expensive and having an excellent catalytic activity. However, when it is used as a main ingredient or, in other words, when it is used in such an amount that it achieves a practical polymerization rate, metal antimony is separated out upon a polycondensation. As a result, darkened parts or contaminated products are generated in the resulting polyester.
As to a catalyst except the antimony compound having an excellent catalytic activity and being capable of preparing the polyester which is free from the above problem, a germanium compound has been put to a practical use already. However, this catalyst has a problem that it is very expensive. In addition, it is apt to be distilled out from a reaction system to an outside during a polymerization. Accordingly, a catalyst concentration in the reaction system may change and a control of the polymerization may be difficult. Therefore, there is a problem in using the germanium compound as a main ingredient of the catalyst.
The polyester produced using a titanium compound represented by tetraalkoxy titanate has a problem that it is apt to be thermally deteriorated upon a melt molding and also that the resulting polyester is significantly colored.
In view of the above circumstances, there has been a demand for a polycondensation catalyst which uses a metal ingredient except antimony, germanium and titanium types as a main metal ingredient of the catalyst; which has an excellent catalytic activity; and which can produce polyester having an excellent color tone and an excellent thermal stability and having an excellent transparency in its molded product.
As to a novel polycondensation catalyst meeting the above demand, a catalytic system consisting of an aluminum compound and a phosphorus compound has been disclosed and is receiving public attentions.
Further, as to a method for producing polyester by the above polycondensation catalytic system, preferred methods have been disclosed (for example, see Patent Documents 1 to 5).
Polyester represented by polyethylene terephthalate obtained in the above polycondensation catalyst system contains only small amount of copolymerized ingredients other than terephthalic acid and ethylene glycol. This polyester has a good color tone, a good transparency and a good heat stability and it meets the above requirements. However, in a case of the copolymerized polyester, a decrease in a polycondensation reaction rate is significant. Accordingly, the color tone of the resulting polyester may be worsened and a molecular weight may be decreased due to a thermal decomposition reaction when a polycondensation reaction is conducted to an extent of an aimed molecular weight.
As to one of methods for enhancing the polycondensation rate, Patent Document 2 mentions a production method wherein terephthalic acid is added to an oligomer prepared by an esterification, then the esterification is conducted once again at a high temperature whereupon an oligomer satisfying a specific viscosity condition, a specific acid value condition, a specific hydroxyl group value condition, a specific molecular weight condition and a specific esterification condition is obtained and the resulting product is subjected to a polycondensation. However, according to the production method of Patent Document 2, terephthalic acid is added to an oligomer prepared by an esterification reaction of dicarboxylic acid with diol and, after that, the esterification is conducted at a high temperature for a long time because of a poor solubility of terephthalic acid. In the copolymerized polyester, as a result of the esterification at a high temperature for a long time, it is difficult to control a composition of a glycerol ingredient and to control a terminal hydroxyl group of the oligomer. In addition, there is another problem that the resulting polyester is apt to be colored due to an affection by a trace of moisture contained in the dicarboxylic acid which is added later and also by oxygen.
In Patent Document 4, it is disclosed that a polycondensation reaction rate can be enhanced when a ratio of a terminal hydroxyl group to total terminal groups of a reaction product prepared by a reaction of a dicarboxylic acid ingredient with a diol ingredient is made within a range of from 55 to 75 molar %. However, in a case of copolymerized polyester having such a complex system that a ratio of a terephthalic acid ingredient to total dicarboxylic acid ingredients is 75 molar % or less and that a diol ingredient is constituted from two or more kinds of diols, it is not possible to efficiently produce the copolymerized polyester having a high molecular weight and a stable composition even if the polycondensation reaction is conducted according to the method mentioned in Patent Document 4.